ath9k: Add RX inactivity detection and reset chip when it occurs

Some ath9k chips can, seemingly at random, end up in a state which can
be described as "deaf". No or nearly no interrupts are generated anymore
for incoming packets. Existing links either break down after a while and
new links will not be established.

The circumstances leading to this "deafness" is still unclear, but some
particular chips (especially 2-stream 11n SoCs, but also others) can go
'deaf' when running AP or mesh (or both) after some time. It's probably
a hardware issue, and doing a channel scan to trigger a chip
reset (which one normally can't do on an AP interface) recovers the
hardware.

The only way the driver can detect this state, is by detecting if there
has been no RX activity for a while. In this case we can proactively
reset the chip (which only takes a small number of milliseconds, so
shouldn't interrupt things too much if it has been idle for several
seconds), which functions as a workaround.

OpenWrt, and various derivatives, have been carrying versions of this
workaround for years, that were never upstreamed. One version[0],
written by Felix Fietkau, used a simple counter and only reset if there
was precisely zero RX activity for a long period of time. This had the
problem that in some cases a small number of interrupts would appear
even if the device was otherwise not responsive. For this reason,
another version[1], written by Simon Wunderlich and Sven Eckelmann, used
a time-based approach to calculate the average number of RX interrupts
over a longer (four-second) interval, and reset the chip when seeing
less than one interrupt per second over this period. However, that
version relied on debugfs counters to keep track of the number of
interrupts, which means it didn't work at all if debugfs was not
enabled.

This patch unifies the two versions: it uses the same approach as Felix'
patch to count the number of RX handler invocations, but uses the same
time-based windowing approach as Simon and Sven's patch to still handle
the case where occasional interrupts appear but the device is otherwise
deaf.

[0] https://patchwork.kernel.org/project/linux-wireless/patch/[email protected]/
[1] https://patchwork.kernel.org/project/linux-wireless/patch/[email protected]/

Signed-off-by: Sven Eckelmann <[email protected]>